Malpositioning of the acetabular component during hip arthroplasty can lead to dislocation, impingement, wear, and reoperation for all of these problems. Proper positioning of the acetabular component during hip arthroplasty requires that the surgeon know the position of the patient's pelvis during surgery so that the component can be inserted in proper position relative to the pelvis. Unfortunately, the position of the patient's pelvis varies widely during surgery and from patient to patient. As a result, large errors in component positioning occur if the surgeon assumes that the patient's pelvis is positioned squarely with the operating table, whether in the lateral or supine position. For example, one study showed that patients' pelvises were malpositioned through a range of 33 degrees around the medial-lateral axis, 47 degrees around the longitudinal axis, and 17 degrees around the anterior-posterior axis. (Chow J C, Eckman K, Jaramaz B, Murphy S, Evaluation of Intraoperative pelvic position during hip arthroplasty using computer tomography/radiography matching, International Society for Computer Assisted Orthopedic Surgery, 2008.)
To reduce the likelihood of component malpositioning, the position of the pelvis can be tracked using computer-assisted surgical navigation techniques, but the majority of surgeons do not employ these techniques. More basic surgical technique involves qualitative assessment of the position of the acetabular component compared to the local surrounding bony and soft tissue anatomy that is visible through the incision. One such technique, for example, uses the transverse acetabular ligament as a qualitative marker of orientation of the native acetabulum. (Archbold H A, Mockford B, Molloy D, McConway J, Ogonda L, Beverland D, The Transverse acetabular ligament: an aid to orientation of the acetabular component during primary total hip replacement: a preliminary study of 1000 cases investigating postoperative stability, J Bone Joint Surg BR. July 2006; 88(7):883-6.)
Unfortunately, such techniques have limited accuracy for several reasons. First, the transverse acetabular ligament is very close to the center of the acetabulum and thus geometrically small errors in interpretation of its position can lead to large angular errors in orientation of the acetabular component. Second, since the transverse acetabular ligament defines a line, which is a two dimensional geometric element: it does not provide true three dimensional guidance. Third, since most hips that are worn are also malformed, taking clues from the local malformed anatomy is a fundamentally flawed concept. Further, even if the local anatomy were not abnormal, as may be the case in patients with femoral head osteonecrosis, it is often the case that the native acetabulum is orientated in a much more vertical position than is appropriate for a prosthetic acetabulum. As a result, while the normally formed native acetabulum may provide clues as to where to position the prosthetic acetabulum, the prosthetic acetabulum should not be placed in the same orientation as the normal native acetabulum.